Water in the Soil - Agroecology: The Ecology of Sustainable Food Systems - page 125

It takes a large amount of water to bring a crop plant to
maturity. For example, a representative crop of corn contain-
ing 10,000 kg/ha of dry matter and having a T ratio of 350
would draw the equivalent of 35 cm of water per ha from the
soil. This moisture must be in the soil at the time the plants
need it, or growth will suffer. Add evaporation losses to this
figure, and it can be seen how moisture is often the most
critical factor in production in moisture-limited regions.
Research focusing on increasing the T efficiency of
crops has begun to show some promise (Davies, 2003;
Shan and Deng, 2003), but even these efforts show little
success in significantly altering the T efficiency ratio.
Without other conditions being limiting, the amount of
water needed to produce a unit of dry matter of a crop
species or variety in a given climate is relatively constant.
This suggests that we need to continue focusing on control
of evaporation from the soil surface.

evaporation rates can be offset by much higher T rates,
depleting soil moisture reserves more rapidly.

Fallow Cropping

In moisture-limited parts of the world such as the Great Plains
of the U.S. and the southeastern wheat belt of Australia,
farmers sometimes alternate between cropping one year and
fallow the next to conserve soil moisture (Figure 9.9). The
elimination of transpirational losses from a crop during the
fallow year allows soil moisture to be stored for the planting
year. Stubble from the previous crop is usually left on the
soil surface during the fallow year to limit evaporative losses,
and then some kind of soil cultivation or herbicide treatment
is used during the fallow season to minimize T losses from
weeds. Alternatively, a pasture crop is sown toward the end
of the cropping year and left as a grazed cover during the
fallow year. Although low rainfall during the fallow year can
cause lower crop yields during the cropping year, a crop
planted following a year of fallow will generally have a
higher yield than if planted without fallow. In fact, as long
as sufficient rainfall for recharge is received during the fallow
year, there is much less risk of crop failure if the crop season
turns out to be a drought year.

ET Efficiency

Since soil itself is quite variable, ET efficiency is also
extremely variable. However, by changing soil and crop
management practices that affect evaporation from the soil
as described below, desirable changes in ET efficiency can
be readily obtained. Ideally, the ratio of transpirational
water loss to evaporative water loss should be as high as
possible. A higher T to E ratio indicates more movement
of water through the plant, and, hence, a higher potential
for production of plant biomass per unit of water used.
Sustainable water management places greatest emphasis,
then, on reducing E so as to have more moisture for T and
related plant growth and development processes.

Managing Surface Evaporation

Evaporation directly from the soil surface normally returns
to the atmosphere more than half the moisture gained from
precipitation. This degree of evaporative loss occurs not
only in dry land regions, but in irrigated arid and rain-fed
humid regions as well. Plant growth suffers as a result of
the loss of moisture through surface evaporation. Any
practice that covers the soil will aid in the reduction of
evaporative losses.

M ANAGING ET

Since T is a plant process that is subject to only minor
control if a plant is otherwise growing normally, it is best
to focus on reducing evaporative loss by managing the
way the plants are grown.

Organic Mulches
A wide range of plant and animal materials can be used to
cover the surface of the soil as mulch in order to reduce
evaporation (and to reduce weed growth and transpirational
losses from the weeds). Commonly used materials include
sawdust, leaves, straw, composted agricultural wastes,
manure, and crop residues (Figure 9.10). Mulches provide
a very effective barrier to moisture loss, and have special
application in intensive garden and small-farm systems, or
with high-value crops such as strawberries, blackberries,
and some other fruit crops (Figure 9.11). Mulches work
best when the cropping system requires only infrequent
cultivation or relies mostly on hand weeding.
Mulching provides a viable option for soil water man-
agement, but at the same time has many other beneficial
effects. It protects the soil from erosion, returns organic
matter and nutrients to the soil, alters the surface reflectivity
(albedo), increases the boundary layer for gaseous diffu-
sion, and allows better infiltration of incoming rainfall. All
of these factors interact.

Crop Choice and Agroecosystem Design

The choice of plant species and the timing of cropping can
influence both T and ET efficiency. Choosing a crop with
less-intensive water needs, such as corn or sorghum, in an
area with very high ET and limited water for irrigation is one
good strategy for soil moisture management. It may also be
useful to shift the growing of more water-intensive crops to
a cooler time of the year when moisture loss potential is less.
Greater vegetative cover can reduce evaporation dra-
matically. One way of gaining more cover is to use inter-
cropping techniques. A forest plantation, for example,
shades the soil surface, whereas an apple orchard with
widely separated rows of trees has much more evaporative
soil surface exposed. But an increase in plant cover (higher
LAI) can also be a liability in drier regions, since lower